Disney’s Finding Nemo made the clownfish a household name, or species even, and drew special notice to its sea anemone home. ‘Ah-ah-ah, you forgot to brush,’ says Nemo’s overprotective father in the excitement of the hero’s first day of school, ‘do you want this anemone to sting you?’

By brushing up against the tentacles of sea anemone, clownfish maintain their immunity to its sting. It is a relationship from which both creatures benefit as ‘obligatory symbionts’, whereby they depend on each other for survival: the clownfish keeps its host clean of harmful invertebrates in return for shelter from predators. It is also a relationship that is being tested, as seas warm and anemones are impacted by more frequent bleaching events.

‘Broadly, if bleaching incidents continue to increase in frequency there is a risk to species survival,’ says researcher Danielle Dixson of the University of Delaware, who has been studying how clownfish behave towards bleaching. ‘Unfortunately our research has shown that bleaching does not break the symbiotic relationship between them.’ Unfortunate, because a bleached anemone is less safe to live in – they increase the chances of being eaten.

Clownfish in bleached host sea anemone (Image: Levent Konuk)

Why are clownfish so stubborn to relocate? ‘Clownfish are behaviourally linked to one or a few particular anemone species for a home,’ says Dixson referring to the fact that 28 species of clownfish each have a preferred host out of ten species of anemone. ‘This limits their ability to acclimate if an entire reef bleaches.’

She tested this behaviour in the lab, offering different clownfish species a choice between a bleached and healthy version of their ideal host. In every case, the fish successfully chose the healthy option, demonstrating that they can detect bleaching. However, when deciding between a bleached version of their preferred host and a non-bleached version of a less-preferred host, they would stick with their preferred, bleached anemone. In other words, in sickness and in health, a clownfish will always choose its parter species as a host.

Out of the lab, it is even harder to up sticks and move in to another host as other anemones are usually spoken for and clownfish schools are restricted by a complex hierarchical system. Though they are all born male, clownfish are sequential hermaphrodites, which means the largest becomes female and second-largest its reproductive mate. The queue to reproduce is in size order. ‘Clownfish can live for decades,’ says Dixson. ‘Imagine if you waited 20 years for your chance to be reproductive, it’s a big deal if someone jumps the line and breaks the order. It’s not like they can just go to the next anemone and be accepted – they are aggressive when it comes to protecting their place.’